Moldable mixture of sacrificial binder and sinterable particulate solids

ABSTRACT

Improvements in molded articles from sinterable particulate solids, improvements in method for making such articles, and novel sacrificial binders for use in making such articles are disclosed. The sacrificial binders used in this invention comprise block polymers having the structural formula 
     
         AB--AB--η A 
    
     wherein &#34;η&#34; is 0 or a positive integer, &#34;A&#34; is a linear or branched polymer that is glassy or crystalline at room temperature and has its softening point in the range of about 80° C. to about 250° C. and &#34;B&#34; is a polymer different from A that behaves as an elastomer at processing temperatures, a plasticizer which may be oil, wax, or oil and wax, and optionally other components.

THE INVENTION

This application is a Continuation of copending U.S. patent applicationSer. No. 905,316, filed May 12, 1978 and now abandoned which wascopending with and a division of U.S. patent application Ser. No.810,464, filed June 27, 1977, now U.S. Pat. No. 4,158,689, which in turnwas a Continuation-in-Part of and copending with U.S. patent applicationSer. No. 644,779 filed Jan. 1, 1976, now abandoned.

This invention relates to improved molded articles from sinteredparticulate solids and to methods and materials for producing articlesfrom particulate materials which exhibit unusual physical integrity inthe green body stage and unusual dimensional precision as finalproducts, i.e., after "burn-out" and sintering. In particular, thisinvention is concerned with articles produced by mixing particulatesolids with a thermoplastic, sacrificial-binder material, molding thearticle into its green body configuration, burning out thesacrificial-binder material and sintering the particulate solids into asingle solid mass, with methods for making such articles and to uniquesacrificial binders for use in making such articles. This invention isapplicable to all particulate solids which are sinterable, as that termis hereinafter defined.

The sacrificial binders of this invention are thermoplastic and containas the principal binder resin a thermoplastic, rubber-related, blockpolymer having the physical properties hereinafter delineated, and thestructural formula AB -AB-.sub.η A, wherein "η" is 0 or a positiveinteger, "A" is a linear or branched polymer that is glassy orcrystalline at room temperature and has its softening point in the rangeof about 80° C. to about 250° C. and "B" is a polymer different inchemical composition from A that behaves as an elastomer at processingtemperatures. A detailed description of block polymers, theirpreparation, composition and physical properties are to be found in"Synthesis of Block Polymers by Homogeneous Anionic Polymerization" byL. J. Fetters, Institute of Polymer Science, The University of Akron,Akron, Ohio published in the Journal of Polymer Science, Part C, No. 26,pages 1-35 (1969) and "Rubber-Related Polymers, I. ThermoplasticElastomers" by W. R. Hendricks and R. J. Enders, Elastomers TechnicalCenter, Shell Development Company, Torrance, Calif., published in RubberTechnology, Second Edition, Chapter 20, pages 515-533, by Van NostrandReinhold Company, New York, Cincinnati, Toronto, London and Melbourne(1973), which are incorporated herein by reference. For the details ofvacuum apparatus and method for performing anionic initiatedpolymerizations which can be used to produce block polymers, see"Procedures for Homogeneous Anionic Polymerization" by Lewis J. Fetters,Journal of Research of the National Bureau of Standards, Vol. 70A, No.5, September-October 1966, pages 421-433 and "The Association ofPolystyryllithium, Polyisoprenyllithium, and Polybutadienyllithium inHydrocarbon Solvents," by Maurice Morton, Lewis J. Fetters, R. A. Pett,and J. F. Meier, Institute of Polymer Science, published inMacromolecules, Vol. 3, pages 327-332, by the American Chemical Society(1970) which are herein incorporated by reference.

Basically, the concept here involved provides for making sinteredarticles from particulate solids employing sacrificial binders whichbehave as thermoplastics during the processing, i.e., mixing andmolding, in that they flow readily at the temperatures used for theseoperations and yet behave in the nature of thermosets during storage ofthe green body at room temperature and during firing until the sinteredbody has taken permanent form. This is achieved with the block polymerelastomers hereinbefore and hereinafter more fully described and the oilor wax or oil and wax used as plasticizer. The oil serves to aid inprocessing by reducing the viscosity of the elastomer which is ofparticular importance during the application of shear forces at mixingand molding temperatures. Thus, when the temperature of the material israised above the glass transition temperature of the block polymerelastomer, i.e., the glass transition temperature of the "A" segments ofthe block polymer, and shear forces are applied, the material becomesless viscous and flows like a thermoplastic. When the system is cooledto room temperature after forming, the "A" segments, e.g., polystyrene,tend to agglomerate to form "domains" and provide a structure similar inphysical behavior to a crosslinked polymer. Subsequent firing at ahigher temperature drives off the oil and/or wax. As no applied shearforces are present during firing, the "A" segment domains remain intheir agglomerated form, thus maintaining the shape of the green bodythrough burn-out and such shape is maintained through sintering by theintegrity of the structure of the residual particulate solids.

DETAILED DESCRIPTION OF THE INVENTION

A. The Principal Binder Resin

The principal binder resin is a thermoplastic block polymer having thestructural formula AB -AB-.sub.η A wherein "η" is 0 or a positiveinteger and "A" and "B" are different polymers. This block polymeradvantageously comprises in excess of 50 wt. % of polymeric material inthe binder excluding the oil and/or wax of the plasticizer. For purposesof simplicity, these polymers will be primarilly discussed withreference to their most simple form wherein "η" is 0, i.e., a blockpolymer of the structural formula A-B-A. It is to be understood that thestatements made about these triblock polymers apply equally to thoseblock polymers wherein "η" is one (1) or greater even though certain ofthe "A" segments will not be terminal and certain of the "B" segmentswill not be center segments.

The "A" segments of these block polymers are non-crosslinked, linear orbranched polymers which are glassy or crystalline at room temperatureand have their softening point in the range of about 80° C. to about250° C. When the molded article is in the green body stage, i.e., afterformation and prior to burn-out of the sacrificial binder, and at roomtemperature, i.e., 20°-25° C., the "A" segments exhibit a modulusgreater than 10⁹ dynes/cm². Where the block polymers are prepared byanionic polymerization, suitable materials for the "A" segments include,but not by way of limitation as one skilled in the art will recognizefrom the physical and chemical characteristics of these and similarpolymers, polystyrene, poly(acrylonitrile), poly (p-bromostyrene),poly(methyl methacrylate), poly(alphamethylstyrene),poly(2-methyl-5-vinylpyridine) and poly(4-vinylpyridine). Other blockpolymers suitable for use in this invention are advantageously preparedby other synthesis routes, i.e., polycondensation, free radicalinitiated polymerization and cationic polymerization using techniquesknown to the art. When these other syntheses are employed, suitablematerials for the "A" segments include, but not by way of limitation asone skilled in the art will recognize from the physical and chemicalcharacteristics of these and similar polymers, poly(vinyl acetate),polyesters, polyamides, polyurethanes, poly(vinyl chloride),polypropylene, polysulfones, poly(phenylene sulfide), poly (4-methylpentene-1) and poly(vinyl alcohol).

The "B" segment of these A-B-A polymers are either rubbery, flexible,glassy or crystalline polymers, as those terms are hereinafter defined,and behave as elastomers at processing temperatures. The "B" segment maybe linear or branched and in some embodiments is chemicallycrosslinkable. In such embodiments, a crosslinking agent therefor isadded during mixing and reacted on molding. When the molded article isin the green body stage and at room temperature, it exhibits a modulusof about 10⁶ -5×10⁷ dynes/cm² when the "B" segment is a rubbery polymer.Where the "B" segment is a flexible polymer, at room temperature, thismodulus will be in the range of about 10⁷ -10⁹ dynes/cm². Where the "B"segment is a glassy or crystalline polymer at room temperature, thismodulus will be above about 10⁹ dynes/cm². Where the block polymers areprepared by anionic polymerization, suitable materials for the "B"segments include, but not by way of limitation as one skilled in the artwill recognize from the physical and chemical characteristics of theseand similar polymers, polybutadiene, polyisoprene,polydimethylbutadiene, poly(ethylene oxide), poly(isopropyl acrylate),poly(octamethylcyclotetrasiloxane), and poly (tetrahydrofuran). Asaforementioned, block polymers suitable for use in this invention areadvantageously prepared by other synthesis routes, i.e.,polycondensation, free radical initiated polymerization and cationicpolymerization. When these other syntheses are employed, suitablematerials for the "B" segments include, but not by way of limitation asone skilled in the art will recognize from the physical and chemicalcharacteristics of these and similar polymers, polyisobutylene, ethylenepropylene rubber, ethylene propylene diene terpolymers, butyl rubber,chloroburyl rubber, bromobutyl rubber, chlorosulfonated polyethylene,epichlorohydrin rubber, fluorocarbon rubbers, silicone elastomers, e.g.,polydimethylsiloxane, polyurethane elastomers and polypropylene oxideelastomers.

The molecular weights of the "A" segments and the "B" segments of theblock polymers suitable for use with this invention will vary with thepolymer segment involved as will be obvious to one skilled in the art inthat physical characteristics must be met as hereinbefore recited. Forinstance, where the block polymer has polystyrene "A" blocks andpolybutadiene "B" blocks, the polystyrene segments advantageously havemolecular weights below about 20,000 and at least two of such segmentshave molecular weights above about 10,000 whereas the polybutadienesegment or segments advantageously have molecular weight or weightsbelow about 80,000 and at least one such segment has molecular weightabove about 40,000. The lower limit of molecular weight for the two "A"blocks is governed by the minimum "A" block chain length required toinsure the formation of a heterogeneous phase while the upper limit of"A" blocks is set by the viscosity of both "A" and "B" blocks when suchviscosity begins to hamper domain formation or processing.

To mix the block polymer with either of the other components of thesacrificial binder or with the particulate solids, the block polymermust be heated to the softening point of the "A" segments or above. Oncethe block polymer has been mixed with the other components of thesacrificial binder, the oil and/or wax can serve as a plasticizer andpermit subsequent working, e.g., molding, etc., at a temperature belowthe softening point of the "A" segments. The lower temperaturelimitations for such working will depend upon the chemical compositionof the "A" segments, the degree to which they are plasticized and theplasticization qualities of the plasticizer. In all cases, however, thelower limit of the working temperatures for such binders will be abovethe temperature at which the "B" segments of the block polymers cease tobehave as elastomers. In general, the mixing temperature isadvantageously in the range between about 15° C. below the softeningpoint of the "A" segments of block polymer used and about 70° C. abovesuch softening point, except where mixing is carried out in the absenceof gaseous oxygen in which case the temperature may be increased toabout 100° C. above such softening point. Thus, the forming temperatureswhich may be used with the various suitable block polymers will varybetween about 65° C. and about 320° C. or 350° C. in the absence of airor other gaseous oxygen. Forming, other than embossing, is carried outat temperatures above the softening point of the "A" segments. Embossingcan be carried out at the same temperatures or even below the softeningpoint of the "A" segments.

In the thermoplastic block polymers having the A-B-A structure, the endsegments, "A", which are rigid at room temperature associate with eachother to give large aggregates which are referred to in the literatureas "domains". At normal handling temperature for the molded articleafter final forming of the green body stage, e.g., room temperature orslightly above, these domains are hard and immobilize the ends of the"B" segments. This end segment immobilization in conjunction with chainentanglements creates physical crosslinks which helps to protect thegreen body from disfiguration as the result of handling. At highertemperatures, the terminal, "A" segments soften and may be disrupted byapplied stress, allowing the polymer to flow. The latter condition makespossibles the mixing, molding, etc. which are necessary or optionalsteps in preparing the green body. Cooling will then provide a greenbody having unusual resistance to physical change prior to the heatingassociated with burn-out and sintering.

Although not essential, it can be advantageous, at least from aprocessing equipment standpoint, to maintain working temperatures duringprocessing such that viscosity is as low as possible. In this regard,viscosity may reach a minimum at a certain temperature but then, uponelevation of temperature, undergoes a viscosity reversal such that theviscosity increases with increasing temperatures. Although dependentupon the nature and amount of the ingredients involved, the temperatureof minimum viscosity in a processing may be ascertained beforehand usingstandard analytic viscosity-temperature measurement equipment. Desirablythen, processing temperatures can be maintained to permit a minimumviscosity and thereby provide easier processing. Usually, for suchprocessing expedience, it is desirable to provide for workingtemperatures near but not in great excess above the temperature whichpermits a desirable minimum viscosity e.g., no higher than 15° F. abovethe temperature for minimum viscosity, more desirably temperatures up tothat temperature at which a viscosity reversal (i.e. increase inviscosity) occurs. It should be noted, however, that apparent viscositymay also show a time dependent behavior due to (a) thixotropic behaviorand (b) irreversible thermally induced degradation. Further behaviortypical of a psuedoplastic fluid is seen with distinct yield pointscharacteristic of Bingham bodies.

B. The Plasticizer

The sacrificial binder also includes a plasticizer which is either anoil or a wax or both. The oils and waxes used for this purpose arenaphthenic, paraffinic or a mixture of paraffinic and naphthenicconstituents. They are sufficiently volatile to be removed easily andrapidly in the burn-out process but insufficiently volatile to besubstantially removed during mixing and/or molding. The loss due tovolatilization during mixing and/or molding is advantageously below 20and preferably below 10 weight percent.

Functionally, the oils and/or waxes must be compatible with the rubberyphase of the principal binder resin when it becomes rubbery onplasticization at a temperature somewhat below the softening point ofthe "A" segments of the principal resin. This gives the binder acapability of accepting high filler loadings while remaining strong andflexible.

At least 75% by weight of the oils used as plasticizers boil in therange of about 550° F. to about 1038° F., preferably in the range ofabout 550° F. to about 865° F. They have viscosities at 210° F. in therange of about 30 to about 220 Saybolt Universal Seconds, hereinafterreferred to as S.U.S., advantageously in the range of about 35 to about155 S.U.S., and preferably in the range of about 35 to about 80 S.U.S.These oils have their Aniline Point in the range of about 170° F. toabout 255° F. The oils may be a product of petroleum refining operationsor vegetable or animal oils and they may include or be low molecularweight synthetic polymers such as polystyrene, poly (alpha-methylstyrene), or a polyolefin. Examples of suitable commercially availableoils include Flexon 580, 680, 765 marketed by Exxon and Shellflex 131,371 and 790 marketed by Shell Chemical Co.

The waxes used have melting points in the range of about 130° F. toabout 170° F. At least about 75% by weight of such wax boils attemperatures in the range of about 600° F. to about 900° F. These may bea product of petroleum refining operations, vegetable or animal waxes orsynthetic polymers such as low molecular weight polyolefins. Examples ofsuitable commercially available waxes include Sunoco Wax 3420, 4412 and4410 marketed by Sun Chemical as well as Paraffin Wax (mp 130° F.)marketed by International Wax Refining.

C. Optional Constituents

The sacrificial binders of this invention may and in certain embodimentsadvantageously do contain additional materials such as supplementaryresins, supplementary elastomers and antioxidants.

Supplementary resins are useful in embodiments where there is a desiredto increase the stiffness of the green body while still providingfluidity at processing temperatures. Suitable secondary resins includeany of the aforementioned polymers suitable for use as "A" segments inblock polymers, resins similar to resins suitable for use as "A"segments and having affinity for the "A" segments of the block polymerused, e.g., cumarone-indene resins and polyindene with block polymershaving polystyrene "A" blocks, and resins which have an affinity for the"B" segment or segments in the block polymers, e.g., polyterpenes withpolybutadiene "B" blocks. It is to be understood that resins having anaffinity for the "A" or "B" segments of the block polymer may also bepolymers suitable for use as "A" or "B" respectively in otherembodiments when they meet the limitations set forth herein for "A" or"B". Examples include Styron A and Styron B marketed by Dow Chemical Co.and Polymel DX-40, a polymerized modified polystyrene naphthenic resin.

Supplementary elastomers are useful in embodiments where there is adesire to improve tear strength in the green body. Suitable secondaryelastomers include natural rubber (e.g. #IRSS) and synthetic elastomers,e.g., polybutadiene, polyisoprene, etc. Supplementary fillers, e.g.,carbon black, may be used in minor amounts to increase the stiffness ofthe green body.

Antioxidants are useful to retard oxidative degradation of the blockpolymer during mixing thus minimizing loss of strength in the greenbody. The antioxidant also allows more rapid removal of binder duringburn-off by minimizing surface oxidation which may tend to seal off thesurface. Suitable antioxidants include, but not by way of limitation,2,6-ditert-butyl-phenol, a polymerized 1,2-dihydro-2,2,4-trimethylquinoline, 2-mercaptobenzimidazole, tetrakis[methylene-3-(3',5'-ditert-butyl-4'-hydroxyphenyl) propionate] methane,etc.

Process aids which are conventional to molding and forming operationswith polymeric materials are likewise useful in the practice of thisinvention to improve the release characteristics of the green body fromany type of molding or forming apparatus with which the green body comesin contact and to improve the flow characteristics of the binderfillermixture during such operations as extrusion molding, injection molding,transfer molding, etc. Process aids which may be of assistance includemethylacetylricinoleate, stearic acid, polyethylene, polyethylene wax,mixtures of natural waxes and wax derivatives, vegetable fats, partiallyoxidized polyethylene, etc.

D. Particulate Material

This invention is applicable to all particulate material that is"sinterable" as that term is hereinafter defined. Specific examplesinclude, ceramic powders such as cordierite powders, alumina,B-spondumene, crystalline or fused silica, mullite, kyanite, zirconia,beryllia, magnesia, titania, chromium oxide, iron oxide and complexoxides comprising two or more of these oxides, metal powders such asiron, iron alloys, copper, copper alloys, aluminum, aluminum alloys,silicon, silicon alloys, nickel base super alloys, cobalt base superalloys and stainless steels, intermetallics such as nickel aluminidesand molybdenum silicide, carbides such as silicon carbide, tungstencarbide, and iron carbide, nitrides such as silicon nitride and boronnitride and composites of metal-ceramics, metal carbides and metalnitrides. Advantageously, average particle diameters exceed 0.01 micronsfor powders and fibers (e.g. chopped magnesium aluminum silicate glassfibers) with length to diameter ratios exceeding 8 can also beadvantageously employed as well as mixtures of these.

One of the advantages of these binders is that they are amenable to highfillings of particulate solids. The molding mixture advantageouslycomprises about 30 to about 70, preferably about 50 to about 65, volumepercent particulate solids with the balance being made up of thesacrificial binders.

E. Proportions of Binder Constituents

The proportions of the principal binder resin or elastomer, i.e., theblock polymer, and the plasticizer, i.e., the oil, wax or oil and wax,may vary widely. In a binder consisting solely of the block polymer andplasticizer, the block polymer will comprise between 10 and 90,preferably between about 30 and about 85, and most preferably betweenabout 45 and about 65, weight percent of the total binder with theplasticizer comprising the balance, i.e., between 90 and 10, preferablybetween about 70 and about 15, and most preferably between about 55 andabout 35 weight percent, provided, however, that the wax constituentwhen used, advantageously does not exceed about 70 weight percent of thebinder.

It will be understood that one may replace any fraction less than 50weight percent, i.e., 0 to between 49 and 50, more commonly betweenabout 0.1 and about 30, weight percent of the block polymer aforedefinedwith an equivalent amount by weight of another (i.e., first replacement)polymer that is within the limitations of "A" in the aforementionedformula.

It will also be understood that one may replace any fraction less than50 weight percent, i.e., 0 to between 49 and 50, more commonly betweenabout 0.1 and about 30, weight percent of the aforedefined block polymerwith an equivalent amount by weight of another (i.e., secondreplacement) polymer that is within the limitations of "B" in theaforementioned formula.

It will be further understood that one may replace any fraction lessthan 50 weight percent, i.e., 0 to between 49 and 50, more commonlybetween about 0.1 and about 40, weight percent of the block polymeraforedefined with an equivalent amount by weight of a block elastomerpolymer having the structural formula X -B (AB)η A]η' wherein "X" is alinking group e.g., derived from a multifunctional halogen functionallinking agent, "A" or "B", "η" is 0 or a positive integer, "η'" is apositive integer greater than 2 and "A" and "B" have generally the samelimitations as "A" and "B" in the hereinbefore described block polymerhaving the structural formula AB -AB-η A, except that "B" of saidelastomer (or said second replacement) polymer desirably also behaves asan elastomer at temperatures between about 5° C. up to about 15° C.below the softening point of "A" of said block polymer aforedefined andabout 90° C. above (desirably about 70° C.) the softening point of "A"of said block polymer aforedefined. The linking agent, when the blockpolymer is made with a linking agent, is a multifunctional (>2) compoundconsisting essentially of elements selected from the group consisting ofcarbon, hydrogen, oxygen, halogens, nitrogen, silicon, phosphorus andsulfur. When anionic polymerization is used, this is a halogenfunctional coupling species. The following are illustrative but notexhaustive: silicon tetrachloride, 1,2,4-tri (chloromethyl) benzene,1,2,4,5-tetra (chloromethyl) benzene, Bis(trichlorosilyl)ethane, cyclictrimer of phosphonitrilic chloride, chloromethylated polystyrene,trichloromethylsilane. The use of these linking agents is discussed inthe aforementioned article "Synthesis of Block Polymers by HomogeneousAnionic Polymerization" by L. J. Fetters. When the block polymers areprepared by other synthesis routes as aforementioned, one would useother linking agents. In the case of free radical polymerization, onemay use a multifunctional compound that will initiate polymerization ofthe "B" block, as when the "B" block is poly (ethyl acrylate), and reactwith the "B" block, e.g., a branched azonitrile such as one prepared byreacting trimethylolpropane with a diisocyanate such as toluenediisocyanate and a glycol such as poly (oxypropylene glycol). In thecase of polycondensation, one may use a multifunctional compound thatwill react with the "B" block, e.g., trimellitic anhydride. In the caseof cationic polymerization, one may react poly(vinyl chloride) withtrimethyl aluminum which will initiate polymerization of the "B" block,as when the "B" block is polyisobutylene, and react with the "B" block.As aforementioned, "X" may be polymer "A", polymer "B" or other linkinggroup. For purposes of this invention and the use of such block polymersin sacrificial binders for a molding mixture, the substitution of alinking group for polymer "A" or polymer "B" as the "X" component doesnot materially affect the physical properties of the block polymer.Linking agents are conventionally used in making such block polymers andare well known to those skilled in the art as shown in the hereinbeforecited literature. With such teachings, selection of a specific linkingagent for a specific block polymer is quite within the skill of thoseskilled in the art.

In combination the substitutions of such "A" type polymers, such "B"type polymers and such other type of block polymers should constituteless than 50 weight percent of the principal binder resin, block polymerpreviously described.

In the following table there is set forth advantageous ranges forconstituents when optional materials are included.

    ______________________________________                                        Sacrificial Binders with Optional Constituents                                            Range,    Preferred  Most Preferred                                           Wt. %     Range Wt. %                                                                              Range, Wt. %                                 Material    of Binders                                                                              of Binder  of Binder                                    ______________________________________                                        block polymer                                                                             10-90     30-85      45-65                                        plasticizer 90-10     70-15      35-55                                         oil        0-90      0-70       0-55                                          wax        0-70      0-30       0-10                                         secondary resin                                                                           0-40      0-25       0-15                                         secondary elastomer                                                                       0-40      0-15       0-10                                         antioxidants                                                                              0-7       0-5        0-3                                          process aids                                                                              0-15      0-10       0-7                                          ______________________________________                                    

F. Definitions

The term "sintering" is used herein to mean the coalescence by heat ofcrystalline or amorphous particles into a solid mass.

The term "holding" is used herein to mean any of the methods of formingknown in the art as extrusion molding, injection molding, compressionmolding, laminating which includes compression molding, transfermolding, pressure molding, displacement molding, blow molding,calendering, and embossing.

The term "processing" is used herein to mean mixing, forming, and mixingand forming.

The term "green body" is used herein to mean a molded article comprisingan intimate mixture of sinterable particulate solids and athermoplastic, organic binder.

The term "molecular weight" is used herein to mean number averagemolecular weight (M_(n)).

The term "room temperature" is used herein to mean a temperature in therange of 20°-25° C.

The term "softening point" is used herein to mean the glass transitiontemperature when used with respect to glassy polymers and thecrystalline melting point when used with respect to crystallinepolymers.

The term "glass transition temperature" is signified herein by thesymbol "T_(g) " and is used herein to mean that temperature below whicha non-crystallizing polymer becomes a supercooled liquid, i.e., a glass.

The term "crystalline melting point" is signified herein by the symbol"T_(m) " and is used herein to mean that temperature at which acrystalline polymer melts and becomes non-crystalline.

Both "glass transition temperature" and "crystalline melting point"represent areas of transition but are practical terms which aresufficiently definitive and exact for the full and complete practice ofthis invention by one skilled in the art without experimentation beyondnormal routine.

The term "glassy polymer" is used herein to mean a non-crystallizingpolymer which at room temperature is below its glass transitiontemperature.

The term "rubbery polymer" is used herein to mean a non-crystallinepolymer that is above its T_(g) at room temperature.

The term "crystalline polymer" is used herein to mean a crystallizingpolymer which is below its T_(m) at room temperature.

The term "flexible polymer" is used herein to mean a polymer which atroom temperature is in transition from glass or crystalline material toan elastomeric state, i.e., to a rubber.

It will be understood by those skilled in the art that it is possiblefor portions of a particular polymeric mass to exist in more than onestate at room temperature and not be in a state of transition from oneto the other, e.g., a polymeric mass in which one portion is a "rubberypolymer" as defined above and a second portion is a "crystallinepolymer" as defined above. Thus, the defined term concerned shall beunderstood to mean that the largest fraction of such polymeric massmeets the limitations of the term used.

The following are illustrative examples wherein, unless otherwisespecified, the materials used are within the limitations hereinbeforeset forth for such materials in the practice of this invention.

EXAMPLE 1

An A-B-A block polymer elastomer, hereinafter called "the elastomer", isprepared by an anionic initiated polymerization using the basic highvacuum apparatus and general procedures for anionic polymerizationdescribed in section 2 (Experimental Techniques) of the aforecitedarticle "Procedures for Homogeneous Anionic Polymerization", by L. J.Fetters. In addition, all attachments of the vessels to the vacuum lineare accomplished through a grease trap as shown in the aforecitedarticle "The Association of Polystyryllithium, Polyisoprenyllithium, andPolybutadienyllithium in Hydrocarbon Solvents" by M. Morton et al.

The reactor is first flamed while under vacuum. The reactor is cooled,sealed off from the vacuum line, and then rinsed with a solution ofethyllithium in n-hexane to react with any residual materials that couldterminate the growing polymer chains. The monomers and solvents to beused in preparing the elastomer are purified according to the article byL. J. Fetters last mentioned above.

The reactor is reattached to the vacuum line. A solution containing0.036 grams of ethyllithium in 3 ml. benzene is added to the reactor. Tothe reactor is charged 370 ml. of benzene. Styrene monomer in the amountof 15 grams is distilled into the reactor through a breakseal onto thetop of the benzene. The contents are cooled to dry-ice/alcoholtemperatures e.g., -65° C. to -78° C. The reactor is sealed off from thevacuum line and the contents allowed to warm-up from a dry/ice alcoholtemperature. As soon as the contents have thawed 0.65 grams of anisolein 4 ml. of benzene is added and shaken with the benzene and styrene inthe reactor. The polymerization of the styrene is allowed to proceed for4 hours at 30° C. The reactor is then reattached to the vacuum line and60 grams of butadiene is distilled in. After the contents have beencooled with liquid nitrogen, the reactor is sealed off from the vacuumline. The mixture is allowed to thaw and after stirring thepolymerization of butadiene is allowed to proceed at 30° C. for 16hours. The mixture is cooled to a dry-ice/alcohol temperature and 15grams of styrene are distilled in after the reactor has been attached tothe vacuum line. The reactor is once again sealed off from the vacuumline, the contents thawed and mixed, and polymerization of the styreneallowed to continue for 4 hours at 30° C. The elastomer in the reactoris then coagulated by slowly pouring the benzene solution into methanolcontaining a small amount of phenylbetanaphthylamine to stabilize theelastomer. The elastomer is dried and is then ready for use as theprincipal binder resin.

This polystyrene-polybutadiene-polystyrene elastomer containing about33.3 wt.% polystyrene in the amount of 14.5 grams is banded on a tightmill which has been preheated to 300° F. About 10 grams, of 100 totalgrams, of glassy cordierite frit are then added on the mill to stabilizethe band. The oil, 12.5 grams of a commercially available, paraffinicpetroleum oil, Flexon 845-a tradename of Exxon Company, U.S.A. isstirred with 50 grams of the cordierite frit. This oil has the followingproperties: specific gravity (60°/60° F.) of 0.8649-0.8811; color (ASTM)of 1-4; viscosity (210° F.) of 43.4-61.5 S.U.S.; aniline point of219°-240° F. and silica gel aromatics of 14.9-16.1 wt.%. About half ofthe oil/cordierite mixture is added to the mill and mixed in. As theviscosity of the mixture decreases, the mill temperature is reduced to290° F. to minimize volatilization of the oil. The remainder of the drycordierite and the remainder of the cordierite/oil mixture are mixed byalternately adding a few grams of one and then of the other to thematerial on the mill. When the mixing is complete in about 20-30minutes, the composition is sheeted from the mill and allowed to cool.

Ribbed sheets of the mixture are prepared by compression molding. Thesheet obtained from the above recited mixing is banded on a 290° F. milland the nip width is decreased so that a sheet 0.030 inch thick isobtained. A preform 31/2 inch square is cut from the 0.030 inch thicksheet, the same providing an excess of material for the ribbed moldbeing used. A press with a 35/8 inch diameter ram and the bottom half ofthe mold are preheated to 250° F. The preform is then placed on thepreheated bottom half of the mold for 15 seconds. The unheated top halfof the mold is then placed upon the preform and the bottom half. Bothhalves of the mold are coated with polytetrafluoroethylene. The press isclosed and a pressure of 2,000 psig is applied. This pressure ismaintained for 15 seconds. The pressure is then released and the ribbedsheet removed from the mold.

The ribbed sheet is then heated in accordance with the following cycle:

                  TABLE I                                                         ______________________________________                                        Step       Temperature, ° F.                                                                   Time, Hrs.                                            ______________________________________                                        1          160          4                                                     2          260          4                                                     3          400-450      4                                                     ______________________________________                                    

The resulting body is then fired in accordance with the following cycle:

                  TABLE II                                                        ______________________________________                                                    Rate of Temp.                                                                             Temperature                                           Step        Rise, ° F./Hr.                                                                     Range ° F.                                     ______________________________________                                        1           600-800     room-2200                                             2           100         2200-2500                                             ______________________________________                                    

This results in a strong, dense, cordierite ribbed sheet.

EXAMPLE 2

The procedures of Example 1 are repeated with the single difference thatthe block polymer elastomer used is a commercially-available triblock(ABA) polymer having a polybutadiene center block and polystyrene endblocks. This block polymer contains 30 weight percent polystyrene, i.e.,Kraton 1101. Kraton is a tradename of Shell Oil Company. This blockpolymer has specific gravity of about 0.95 and intrinsic viscosity ofabout 1.00 dl/g (30° C. in toluene).

This results in a strong, dense, cordierite ribbed sheet.

EXAMPLE 3

The procedures of Example 2 are repeated except for the difference thatthe elastomer, the oil and the frit are first stirred together and thenmixed in a Banbury mixer which is preheated to 310° to 320° F. for 15minutes. The resulting mixture is then blended on a two roll mill whichhas been preheated to 290° F. After two or five minutes of mill mixing,the composition is sheeted and allowed to cool. It is now ready forcompression molding and is molded in accordance with the previousexamples. This results in a strong, dense, cordierite sheet.

EXAMPLE 4

The procedures of Example 3 are repeated except for the followingdifferences: twenty-nine (29.0) parts by weight of thepolystyrene-polybutadiene-polystyrene triblock polymer elastomer aremixed with 27.3 parts by weight of paraffin wax (m.p. 130° F.), and 200parts by weight of glassy cordierite frit. This results in a strong,dense, cordierite sheet. The paraffin wax acts as a process aid andplasticizer and provides smoother extrusions. It also increases thestiffness of the green body.

EXAMPLE 5

The procedures of Example 3 are repeated except for the followingdifferences: test bars are molded using a ram type injection moldingmachine. The barrel and nozzle of the injection molding machine arepreheated and controlled to 320° F. A mold for molding two test bars ispreheated to 100° F. and clamped with a pressure of 15,000 psi. Twenty(20) grams of pieces cut from the above sheet are introduced into thebarrel and allowed to heat for 5 minutes. The material is then injectedinto the mold with the ram pressure (800 psi) being maintained for oneminute. Ram pressure is released followed by release of clamp pressurefrom the mold and the test bars are removed from the mold. The test barsare first heated and then fired using the same cycles used for the sheetmaterial in the preceding examples. This results in strong, dense,cordierite bars.

EXAMPLE 6

The procedures of Example 2 are repeated except for the followingdifferences: A flat sheet is prepared by use of a screw type extruderhaving a 2-inch bore. The binder cordierite mixture is pelletized andthe pellets are fed into the hopper of the extruder. It is then conveyedthrough the extruder and passed through a thin slit die (0.020 inchthick, 4 inches wide). The temperature settings on the extruder are:feed section 175° F., transition section 250° F., and die section 300°F. The sheet is then cooled to room temperature and stored. The cooledsheet is flexible and suitable for subsequent handling such as slitting,rewinding and embossing and when fired yields a strong, dense,cordierite sheet.

EXAMPLE 7

The procedures of Example 2 are repeated with the exception that adifferent, commercially-available,polystyrene-polybutadiene-polystyrene, triblock polymer is used as theelastomer, i.e., Kraton 1102. This elastomer contains about 28% byweight polystyrene and has a lower viscosity, i.e., intrinsic viscosityof 0.84, dl/g (30° C. in toluene), than the one previously exemplifiedand identified by containing 30% by weight polystyrene. As a result ofthe lower viscosity of the elastomer, the binder composition has a lowerviscosity and is more easily processed. The product obtained is not asstiff in the green state but still results in a strong, dense,cordierite ribbed sheet when fired.

EXAMPLE 8

The procedures of Example 2 are repeated with the single difference thata commercially-available, polystyrene-polyisoprene-polystyrene triblockpolymer elastomer, i.e., Kraton 1107 containing about 14 weight percentpolystyrene is used as the elastomer. When mixed with oil and frit, thiselastomer yields a composition of lower modulus but of inferior strengthin the green body state to those of the preceding examples. It stillyields, however, a strong, dense, cordierite ribbed sheet after firing.

EXAMPLE 9

The procedures of Example 2 are repeated except for the differences: acommercially-available, triblock polymer elastomer having polystyreneend blocks and a poly (ethylene-butylene) rubber center and containing12-14 weight percent polystyrene, i.e., Kraton GX-7820, is used as theelastomer and a paraffinic oil, i.e., Shellflex 790, is substituted forthe oil used in Example 2. Shellflex is a tradename of Shell ChemicalCompany. This change of oils is made to reduce the volatilization whichwould otherwise occur because of the higher mill temperatures bestsuited to work this plasticized elastomer, i.e., about 350° F. The greenbody is stiffer than that of Example 2 and a poorer dispersion of filleris obtained. The ribbed sheet obtained after firing is less dense and oflesser strength than those produced in the preceding examples.

EXAMPLE 10

The procedures of Example 2 are repeated except for the difference thatthe parts by weight of the elastomer, oil and cordierite frit are asfollows: elastomer 2.75, oil 22.77 and frit 100.00. This exampleillustrates approaching toward minimum elastomer and maximum oil contentin the binder. The green body is soft and lacking in strength andintegrity. The ribbed sheet obtained after firing is less dense and oflesser strength than those produced in the preceding examples.

EXAMPLE 11

The procedures of Example 2 are repeated except for the difference thatthe parts by weight of the elastomer, oil and cordierite frit are asfollows: elastomer 24.78, oil 2.53 and frit 100.00. This exampleillustrates approaching toward maximum elastomer and minimum oil contentin the binder. This composition is stiffer and more difficult to processthan those produced in the preceding examples.

EXAMPLE 12

The procedures of Example 2 are repeated except for the difference thatparaffin wax (m.p. 130° F.) is substituted for the oil in the binder andthe parts by weight of the elastomer, wax and frit are as follows:elastomer 8.26, wax 19.07, and frit 100.0. This composition results in astiffer green body with less tendency to crack during burn-out andfiring.

EXAMPLE 13

A commercially-available polystyrene in the amount of 12.17 grams isbanded on a tight mill which has been preheated to 320° F. Thispolystyrene has specific gravity of about 1.05, a Vicat softening pointof about 97° C., and a melt flow of about 3.5 grams/10 minutes(ASTM-D123 8 condition G). The polystyrene-polybutadiene-polystyreneelastomer of Example 2 in the amount of 12.39 grams (30 weight percentpolystyrene) is then combined on the mill with the polystyrene after thetemperature is reduced to 310° F. for the remainder of the mixing. Twograms. of the 100 grams of glassy cordierite frit to be used, are thenadded on the mill to stabilize the band. The paraffinic petroleum oil ofExample 2 in the amount of 3.80 grams is stirred with 50 grams of thecordierite frit. About half of the oil/cordierite mixture is added tothe mill and mixed in. The remainder of the dry cordierite and thecordierite/oil mixture are mixed in by alternating a few grams of oneand then the other. When mixing is complete in about 20 to 25 minutes,the composition is sheeted from the mill and allowed to cool. Theremainder of the processing is the same as in Example 2. This alsoresults in a stiff green body but with improved green strength andbetter retention of shape during burn-out. The processability of the miximproved as mixing proceeded.

EXAMPLE 14

Natural rubber (#1 ribbed smoke sheet) in the amount of 10.67 grams isbanded on a tight mill which has been preheated to 300° F. It is thenremoved from the mill and set aside. The elastomer of Example 2 in theamount of 13.77 grams is then banded on the 300° F. mill. Half of thenatural rubber is then added to the mixture on the mill. The remainderof the dry cordierite and the natural rubber are then mixed in byalternating a few grams of one and then of the other. When mixing iscomplete in about 20-25 minutes, the composition is sheeted from themill and allowed to cool. The remainder of the processing is the same asthat of Example 2. The use of the natural rubber gives improved tearstrength in the green body and gives a modulus lower than that inExample 2.

EXAMPLE 15

The procedures of Example 2 are repeated with the following changes: allof the cordierite is added in the first addition and none is mixedseparately with the oil. In addition, the parts by weight of elastomer,oil and cordierite are as follows: elastomer 27.54, oil 25.30 and frit2.72. This provides an extremely soft composition with a tendency toprovide a porous ceramic part.

EXAMPLE 16

The procedures of Example 2 are repeated with the following changes: theparts by weight of elastomer, oil and frit are elastomer 12.39, oil13.92 and frit 123.81. At this high loading of frit, the green body isvery stiff but very weak and difficult to process.

EXAMPLE 17

The procedure of Example 2 is repeated with the following changes: theparts by weight of elastomer, oil and frit are elastomer 13.77, oil12.53 and frit 100.00 In addition, the mixture also contains 0.14 partsby weight of an antioxidant, i.e., 4,4'-methylenebis(2,6-di-tert-butyl-phenol). Use of the antioxidant results in betterretention of strength during mixing and improved resistance to slumpingduring binder burn-off.

EXAMPLE 18

The procedure of Example 2 is repeated with the following changes: theproportions by weight of the binder ingredients and frit are elastomer13.77, oil 11.39, frit 100.00 and methylacetylricinoleate, a processaid, 1.36. Use of the process aid results in better processabilityduring calendering and extrusion.

EXAMPLE 19

Sintered, modified, beta-alumina tubes are produced by extrusion moldingusing the elastomer and oil of Example 2, wax, an antioxidant andparticulate beta-alumina. The composition mixed for molding is asfollows:

                  TABLE I                                                         ______________________________________                                        Material            Parts by Weight                                           ______________________________________                                        Elastomer of Example 2                                                                            4.7                                                       Oil of Example 2    3.2                                                       Paraffic wax (m.p. 135° F.)                                                                3.5                                                       Antioxidant, poly 1,2-dihydro-                                                 2,24-trimethylquinoline                                                                          0.5                                                       Powder lithium-modified beta-                                                  alumina (9.0% Na, 0.8% Li.sub.2 O                                             and 90.2% Al.sub.2 O.sub.3)                                                                      50.0                                                      ______________________________________                                    

The above-listed materials are mixed at 310° F. (154° C.) on a two rollmill After being extruded into tube shape using a 310° F. (154° C.)nozzle temperature with a barrel temperature of 340° F. (171° C.), thetubes are then burned-out and sintered using the following schedule:

                  TABLE II                                                        ______________________________________                                                 Rate of Temp. Rise Temperature                                       Step     ° C./Hr.    Range, ° C.                                ______________________________________                                        1        50                 100-700                                           2        Hold for 15 hrs.   700                                               3        Cool to room temperature                                                      over 3 hours                                                         ______________________________________                                    

Each such tube is placed in a platinum tube which is mechanicallysealed. The sealed sample is then placed in a furnace. The furnace isheated to 1100° C. over 15 hours. From 1100° C., the furnace is heatedto 1570° C. over 4 hours and 45 minutes where it is held for 15 minutesand then cooled to 1420° C. over 15 minutes. The sample is held at 1420°C. for 8 hours and then cooled to 360° C. over 7 hours at which pointthe sample is allowed to cool to ambient temperature in air.

EXAMPLE 20

The procedures of the previous examples are repeated with the singledifference that particulate alumina is substituted for the cordieriteand sintering is carried out at temperatures of 1600°-1700° C.

EXAMPLE 21

The procedures of the previous examples are repeated with the singledifference that particulate silicon carbide is substituted for thecordierite, and sintering is carried out at temperatures of 2000°-2200°C.

EXAMPLE 22

The procedures of the previous examples are repeated except thatparticulate aluminum is substituted for the cordierite and sintering iscarried out at temperatures of 550°-650° C.

EXAMPLE 23

The procedures of the previous examples are repeated except thatparticulate stainless steel is substituted for the cordierite andsintering is carried out at temperatures of 1400°-1500° C.

EXAMPLE 24

The procedures of the previous examples are repeated with fibers havinga length to diameter ratio exceeding 8 (4D>8) of the respective materialused to replace powders of such materials in those examples usingpowders.

EXAMPLE 25

The procedures of Example 2 are repeated with the following changes: theparts by weight of the binder ingredients and frit are elastomer 84.6and frit 34.5. No attempt is made to prepare ribbed sheet or to preparea strong, dense cordierite body. The purpose of preparing thiscomposition is to determine if the minimum in viscosity as a function oftemperature is an inherent feature of the processing characteristics ofthe block polymer elastomer based binder compositions. When theprocessing characteristics of the mixed composition are examined using acapillary rheometer, the viscosity initially decreases with increase intemperature and then increases with increase in temperature. The minimumin viscosity for this composition occurs at about 220° C. It is expectedthat the particular temperature at which a minimum in viscosity occurswill vary from composition to composition and be dependent on thespecific ingredients and their proportions in a particular composition.

The foregoing examples are merely illustrative of the practice of thisinvention and those skilled in the art will readily recognize thatmodifications and variations may be made in these examples withoutdeparting from the scope of this invention as set forth in the appendedclaims.

What is claimed is:
 1. In a moldable mixture for preparing sinteringarticles which consists essentially of about 30 to about 70 volumepercent of sinterable particulate solids and about 70 to about 30 volumepercent of an organic sacrificial binder, the improvement wherein saidorganic sacrificial binder consists essentially of an intimate mixtureof about 10 to about 90 parts by weight of resinous material and about90 to about 10 parts by weight of a plasticizer for said resinousmaterial wherein (1) said resinous material is a block polymer havingthe structural formula AB-AR-η A, wherein "η" is 0 or a positiveinteger, "A" is a linear or branched polymer that is glassy orcrystalline at 20°-25° C., has its softening point in the range of about80° to about 250° C. and "B" is a polymer of different chemicalcomposition than A that behaves as an elastomer at temperatures betweenabout 15° C. below the softening point of A and about 100° C. above thesoftening point of A, and(2) said plasticizer is selected from the groupconsisting of(a) an oil at least 75 percent by weight of which boils inthe range of about 550° F. to about 1038° F., has viscosity at 210° F.in the range of about 30 to about 220 Saybolt Universal Seconds and ananiline point in the range of about 170° F. to about 255° F. (b) a waxmelting at a temperature in the range of about 130° F. to about 170° F.at least 75 percent by weight of which boils at temperatures in therange of about 600° F. to about 900° F., and (c) an oil in accordancewith (a) and a wax in accordance with (b)and wherein 0 to between 49 and50 weight percent of said block polymer is replaced by a firstreplacement polymer meeting the limitations of polymer "A" in saidstructural formula, and 0 to about 49 and 50 weight percent of saidblock polymer is replaced with a second replacement polymer meeting thelimitations of polymer "B" in said structural formula, the sum of saidreplacements of said block polymer comprising in combination less than50 weight percent of said block polymer.
 2. A moldable mixture inaccordance with claim 1 wherein "B" of said block polymer behaves as anelastomer at temperatures between about 15° C. below the softening pointof "A" and about 70° C. above the softening point of "A".
 3. In amoldable mixture for preparing sintered articles which consistsessentially of about 30 to about 70 volume percent of sinterableparticulate solids and about 70 to about 30 volume percent of an organicsacrificial binder the improvement wherein said organic sacrificalbinder consists essentially of an intimate mixture of about 10 to about90 parts by weight of resinous material and about 90 to about 10 partsby weight of a plasticizer for said resinous material wherein(1) saidresinous material is a block polymer having the structure formulaAB-AB-η A, wherein "η" is 0 or a positive integer, "A" is a linear orbranched polymer that is glassy or crystalline at 20°-25° C., has itssoftening point in the range of about 80° to about 250° C. and "B" is apolymer of different chemical composition than A that behaves as anelastomer at temperatures between about 15° C. below the softening pointof A and about 100° C. above the softening point of A, and (2) saidplasticizer is selected from the group consisting of(a) an oil at least75 percent by weight of which boils in the range of about 550° F. toabout 1038° F., has viscosity at 210° F. in the range of about 30 toabout 220 Saybolt Universal Seconds and an aniline point in the range ofabout 170° F. to about 2550° F.; (b) a wax melting in a range of about130° F. to about 170° F. at least 75 percent by weight of which boils attemperatures in the range of about 600° F. to about 900° F., and (c) anoil in accordance with (a) and a wax in accordance with (b) andwherein 0to between 49 and 50 weight percent of said block polymer is replaced bya first replacement polymer meeting the limitations of polymer "A" insaid structural formula, and 0 to between 49 and 50 weight percent ofsaid block polymer is replaced with a second replacement polymer meetingthe limitations of polymer "B" in said structural formula, and 0 tobetween 49 and 50 weight percent of said block polymer is replaced withan elastomer polymer having the structural formula X-B (AB).sub.ηA].sub.η ' wherein "η" is 0 or a positive integer, X is a linking group,A or B, "η'" is a positive integer greater than 2, "A" and "B" have thesame limitations as A and B of said block polymer except that "B" ofsaid elastomer polymer behaves as an elastomer at temperatures betweenabout 5° C. below the softening point of "A" of said block polymer andabout 90° C. above the softening point of "A" of said block polymer, thesum of said replacements of said block polymer comprising less than 50weight percent of said block polymer.
 4. A moldable mixture inaccordance with claim 3 wherein said organic sacrificial binder consistsessentially of an intimate mixture of about 30 to about 85 parts byweight of said resinous material and about 70 to about 15 parts byweight of said plasticizer.
 5. A moldable mixture in accordance withclaim 3 wherein said organic sacrificial lanider consists essentially ofan intimate mixture of about 45 to about 65 parts by weight of saidresinous material and about 55 to about 35 parts by weight of saidplasticizer.